1. Field of the Invention
The present invention relates to a combustor, a burner, and a gas turbine.
2. Description of the Related Art
Fuel having a calorific value lower than that of liquefied natural gas (LNG) which is popular fuel for gas turbines is hard to be burnt generally because of low flame temperature and lower burning velocity. However, such fuel, i.e., low calorific gas is characterized by a small amount of NOx emissions during burning. Examples of such low BTU gas typically include blast furnace gas (BFG). Blast furnace gas is side product gas which is produced by a blast furnace in an iron manufacturing process. In recent years, there has been a growing need for blast furnace gas as gas turbine fuel. However, the blast furnace gas is incombustible because of containing a large amount of N2 and CO2 in addition to carbon monoxide (CO) and hydrogen (H2) which are main constituents. Thus, it is difficult for a gas turbine to operate on mono-fuel combustion using blast furnace gas in a range from ignition to a full load. To stably operate a gas turbine (to stably burn blast furnace gas) in a range from ignition to a partial load, pilot fuel for start-up is additionally needed.
Examples of low BTU gas include also gasification gas such as coal or biomass (woodchips or the like) in addition to blast furnace gas. There is a growing need for fuel created from coal or the like as fuel for gas turbines in view of the efficient use of resources. However, such fuel created from coal or the like is incombustible gas containing a large amount of N2; therefore, naturally, pilot fuel is additionally needed.
Because of this, in order for a combustor to achieve flame stabilization of incombustible gas, it is general to adopt diffusion combustion in which fuel and air are supplied from respective different flow passages and to configure a burner capable of burning dual fuel consisting of pilot fuel (e.g., liquid fuel) and low BTU gas. As one example, JP-5-86902-A describes a burner in which a liquid fuel nozzle is disposed at a radially central portion, with the liquid fuel nozzle designed to operate in a range from the start to partial load of a gas turbine, and gas jet holes are arranged on the outer circumference of the liquid fuel nozzle.
On the other hand, high calorific gas such as LNG or the like has high flame temperature; therefore, it is necessary to devise a reduction in the amount of NOx emissions. Examples of a combustion method for reducing the amount of NOx emissions include distributed lean burn. This distributed lean burn is a combustion system as below. Fuel and air are coaxially jetted toward air holes installed in a plate. Contraction flow at air flow inlets and turbulence due to abrupt expansion at air flow outlets are used to rapidly mix the fuel with air in a short distance and supply them into a combustion chamber (refer to JP-2003-148734-A). The distributed lean burn system has a short mixing length of fuel and air; therefore, it is expected to produce an effect of promoting low NOx emissions even if not only LNG but hydrogen-containing fuel having high burning velocity is used.